Catalytic converter

✓What is a catalytic converter?
A catalytic converter converts the harmful exhaust gases produced by the engine into less harmful emissions before they leave the vehicle's exhaust system.
The catalytic converter looks similar to a muffler. It is typically a stainless steel housing with a ceramic honeycomb inside that is impregnated with platinum, palladium, and rhodium. These elements help clean up harmful exhaust gases that otherwise would be expelled from the tail pipe.

It is common for a four-cylinder engine to be equipped with one or two catalytic converters, while six and eight-cylinder engines can have up to four. 

✓How does the catalytic converter work?
The catalytic converter has multiple sections, commonly referred to as beds. Each contains specific catalyst elements that work at a temperature of at least 400 degrees F to promote specific chemical reactions. This converts harmful CO, HC, and NOx exhaust gases into less harmful CO2, H2O, and N gases that are released from the vehicle's exhaust pipe.

The first section of the catalytic converter is called the reduction bed and is coated with rhodium. It is called the reduction bed because its purpose is to reduce the NOx gases back in to harmless nitrogen and oxygen.

The next section of the catalytic converter is the oxygen storage bed, which is coated with cerium. Its purpose is to maintain an ideal level of oxygen for use by the rear of the converter. It does this by storing and releasing the oxygen that gets released from the reduction of NOx in the previous reduction bed.

The oxygen is then available for use in the final oxidizing bed, which is coated with platinum and paladium. The purpose of the oxidizing bed is to complete the combustion of CO by adding oxygen. The oxidizing bed also uses oxygen to burn any of the raw HCs that still remain in the exhaust gases.

✓What are the symptoms related to a bad catalytic converter?
A failing catalytic converter can cause a vehicle to fail emissions tests and the Check Engine Light to illuminate. The exhaust might have an unusual smell. A "restricted" catalytic converter can result in a loss of engine power.
✓Symptoms of a Bad Catalytic Converter
A catalytic converter is an exhaust emissions control device used in both gasoline and diesel engines. Often referred to as a "cat," its job is to convert a vehicle's exhaust emissions into a less harmful gas before the exhaust exits the tailpipe.
A catalytic converter is a metal canister installed in the exhaust system near the engine. It contains precious metals such as platinum, palladium and rhodium. When raw exhaust flows through it, multiple chemical reactions occur. The final product is an exhaust that's free of nitrogen oxides, carbon monoxide and hydrocarbons, three very harmful gases not only to the environment, but also to humans.
✓Signs of a bad catalytic converter
An illuminated check engine light. Your vehicle's engine computer receives feedback from sensors placed in the exhaust system called oxygen sensors. One sensor gets placed before the cat, and is called the "upstream" oxygen sensor. The other is placed after the cat, and is called the "downstream" oxygen sensor.

Since the job of the catalytic converter is to filter out harmful gases from the engine's exhaust, the reading of the two sensors should be different. If they are the same, then that indicates the catalytic converter is not doing its job. In this situation your check engine light should come on and will set a code for an inefficient catalytic converter.
A failed vehicle emissions test. Some areas in the U.S. require a diagnostic check of the engine's computer to pass the emissions test. If a car has a bad catalytic converter, a trouble code will be stored in the computer, and the vehicle will not pass the test.
An unusual exhaust smell. Gasoline contains sulfur that turns into hydrogen sulfide during engine combustion. The catalytic converter changes this into sulfur dioxide, which is odorless. An inefficient or failing cat can leave more unburnt fuel in the exhaust which will smell like sulfur or rotten eggs.
A lack of engine power. In some cases, a catalytic converter will become plugged up and cause an exhaust restriction. This is normally caused by unburnt fuel igniting in the catalyst material and causing it to deteriorate. The exhaust restriction will severely impact the power output of the engine and require more throttle input to move the car. Eventually, the cat can completely block exhaust flow, and the engine will not run.
A rattling noise under the vehicle. The catalyst material is shaped like a honeycomb inside of its metal casing. A failing cat can break apart and cause a rattling noise while the engine is running. If it's not replaced, parts of the catalyst can break off and get stuck in the vehicle's muffler or resonator further down the exhaust stream.
✓Symptoms of a Bad Catalytic Converter
A catalytic converter is an exhaust emissions control device used in both gasoline and diesel engines. Often referred to as a "cat," its job is to convert a vehicle's exhaust emissions into a less harmful gas before the exhaust exits the tailpipe.

A catalytic converter is a metal canister installed in the exhaust system near the engine. It contains precious metals such as platinum, palladium and rhodium. When raw exhaust flows through it, multiple chemical reactions occur. The final product is an exhaust that's free of nitrogen oxides, carbon monoxide and hydrocarbons, three very harmful gases not only to the environment, but also to humans.

catalytic converter
Catalytic converter

✓Signs of a bad catalytic converter
An illuminated check engine light. Your vehicle's engine computer receives feedback from sensors placed in the exhaust system called oxygen sensors. One sensor gets placed before the cat, and is called the "upstream" oxygen sensor. The other is placed after the cat, and is called the "downstream" oxygen sensor.

Since the job of the catalytic converter is to filter out harmful gases from the engine's exhaust, the reading of the two sensors should be different. If they are the same, then that indicates the catalytic converter is not doing its job. In this situation your check engine light should come on and will set a code for an inefficient catalytic converter.
A failed vehicle emissions test. Some areas in the U.S. require a diagnostic check of the engine's computer to pass the emissions test. If a car has a bad catalytic converter, a trouble code will be stored in the computer, and the vehicle will not pass the test.
An unusual exhaust smell. Gasoline contains sulfur that turns into hydrogen sulfide during engine combustion. The catalytic converter changes this into sulfur dioxide, which is odorless. An inefficient or failing cat can leave more unburnt fuel in the exhaust which will smell like sulfur or rotten eggs.
A lack of engine power. In some cases, a catalytic converter will become plugged up and cause an exhaust restriction. This is normally caused by unburnt fuel igniting in the catalyst material and causing it to deteriorate. The exhaust restriction will severely impact the power output of the engine and require more throttle input to move the car. Eventually, the cat can completely block exhaust flow, and the engine will not run.
A rattling noise under the vehicle. The catalyst material is shaped like a honeycomb inside of its metal casing. A failing cat can break apart and cause a rattling noise while the engine is running. If it's not replaced, parts of the catalyst can break off and get stuck in the vehicle's muffler or resonator further down the exhaust stream.
✓How to troubleshoot a catalytic converter
To determine if your catalytic converter has failed, a professional technician is needed. The tech will begin with a physical inspection and check to see if the catalyst material has broken apart, causing a rattling noise. In a situation of engine power loss, an exhaust system backpressure test will be performed. In some cases, a computer diagnostic can inform a tech of a poor performing cat through data relayed from sensors placed before and after the cat, called oxygen sensors.

Almost all catalytic converter failures are due to an issue with the engine. An engine that is not performing like normal can ruin a cat by dumping too much raw fuel into the exhaust. Some common items that can cause this issue would be faulty fuel injectors, a bad ignition coil, fouled spark plugs and a leaking fuel pressure regulator.

A flashing check engine light indicates a catalyst damaging misfire and this can damage a cat very quickly. In this situation, the engine should be shut off immediately, and the car should be towed to a repair shop for diagnosis.
It is very important that the cause of the cat failure is determined before a new cat is installed. Failure to do so may ruin a new catalytic converter, and the warranty may not cover the damaged part.
✓Beware of catalytic converter theft
Catalytic converters are often stolen because they contain expensive materials. A thief can remove a cat in a matter of minutes, so the police should be called immediately as the thieves may still be in the area stealing more of them. A sign of a missing catalytic converter is a very loud exhaust note, and some insurance policies will cover the replacement.

✓Can I drive with a catalytic converter problem?
Unless the Catalytic Converter becomes restricted or “clogged” you should be able to drive the vehicle. If black smoke in noted from the exhaust you should not drive the vehicle. This would indicate unburned fuel entering the converter which could damage it.

✓How often do catalytic converters need to be replaced?
If the engine is running properly, the catalytic converter should last the life of the vehicle. Failure is commonly caused by too much raw fuel entering the converter, and example of which would be the result of an engine misfire. Too much unburned fuel can coat the catalyst causing the converter to stop working, the converter can also overheat and break apart inside becoming restricted or "plugged".

Your catalytic converter is covered by the vehicle's emissions warranty, which extends beyond the manufacturer's factory warranty.  Along with other emission control devices, federal law requires manufacturers to replace a defective cat for up to 15 years or 150,000 miles. Because this coverage varies by state and vehicle type, refer to your owner’s manual for specific warranty information.

✓How are catalytic converter issues diagnosed?
Depending on the type of failure, different diagnostic approaches are necessary. For a lack of power concern, a back pressure test should be performed to check for a plugged converter. If an illuminated check engine light leads to a fault code that indicates a failing converter, the manufacturer recommended diagnostic procedures should be followed to confirm the fault. A catalytic converter could also have a noise concern (exhaust leak or rattling type noise). In this case, diagnosing the issue would involve duplicating the noise and confirming in is coming from the catalytic converter.

✓How is a catalytic converter replaced?
A failed catalytic converter will require replacement. A vehicle may have more than one converter, it is very important to make sure the correct converter is identified when ordering parts and installing the new converter. Most converters are bolted in place in the exhaust system. If not bolted, the converter would be welded or clamped in place. A bolted in converter can be replaced by unbolting (penetrating oil should be used prior to attempting to remove any exhaust system bolts) the old converter and bolting the new one in. If a converter is welded in place, the old one will need to be cut out and the new converter installed - either by welded or using exhaust pipe clamps as required.

Catalytic converters are normally placed near the engine. Some are built directly into the exhaust manifold as they work most efficiently when hot.  Some cats can exceed 1,000 degrees, so never touch a catalytic converter while an engine is running or after its been shut off. Many hours are required before a catalytic converter is serviceable, so proceed with caution anytime you’re working with your exhaust system.

✓OBD codes related to Catalytic Converter Replacement
P0420: Catalyst System Efficiency Below Threshold
P0421: Warm Up Catalyst Efficiency Below Threshold
P0422: Main Catalyst Efficiency Below Threshold
RepairPal Recommendations for catalytic converter issues
Care should always be taken when working around hot exhaust components. An engine not running properly is nearly always the cause of a catalytic converter failure. Engine operation should the thoroughly checked after replacing a faulty converter. Failure to do so could result in failure of the new converter.

Catalytic converters are vehicle-specific, and special attention needs to be paid to the quality of parts used. Cheaper cats are to be used with extreme caution, as they may not perform adequately for vehicle emission tests and inspections.  Some states like California allow only certified cats to be used for replacement.  A High-quality Original Equipment Manufacturer (OEM) replacement CAT is highly recommended.

✓What to look out for when dealing with catalytic converter issues
Rust build up on exhaust system components is not uncommon, penetrating oil should be used on threads prior to attempting removal. A new Catalytic Converter can be quickly damaged if installed in a vehicle with a poorly running engine. Proper diagnoses should always be following prior to replacing a catalytic converter, just because a fault code points to the converter, does not guarantee replacing it will correct the problem. Many vehicles have more than one converter, care should be taken to make sure the correct converter is diagnosed or replaced.

✓Can I replace the catalytic converter myself?
A Catalytic Converter can either be bolted or welded in place. In most cases a converter bolted in place can be removed using basic hand tools (be sure to use penetrating oil before attempting to remove exhaust system components). If the converter is welded in place, some sort of cutting device will be necessary to remove it. Be sure to check your vehicle to see what you have before attempting this repair. Having the necessary tools on hand before starting will help the job run smoothly. It is very important to reinstall any heat shields that were removed in order to replace the converter because of the heat generated during normal operation.